This application relates to and incorporates herein by reference Japanese Patent Application No. 11-168391 filed on Jun. 15, 1999.
The-present invention relates to mobile communications and devices therefor, which transmits a self-identification code upon entering into a communication service area of an immobile communication device of a fixed station and executes link processing for communications with the immobile communication station, thereby executing communications with the self-identification code after establishing the link.
Inter-vehicle communication systems execute dedicated short-range communications (DSRC) for communications between a mobile communication device mounted on a mobile object such as a vehicle and an immobile communication device such as a roadside station fixed on the roadside. Those systems may be applied to electronic toll collection systems (ETC) which collect traffic fees at toll roads such as expressways.
In each of those systems, the mobile device transmits a link-identification code provided exclusively thereto when entering into the communication service area of the immobile device to establish a communication link with the immobile device. The mobile device then communicates with the immobile device within the slot of a communication frame allocated thereto, after the link is established. According to this communication, even under the condition that a plurality of mobile communication devices are in the communication service area, the communication processing can be executed separately within the allocated slots with the respective link-identification codes. Thus, one-to-multiple bi-directional communication can be effected.
In the above inter-vehicle communication system, the link-identification code (LID) used between the mobile device and the immobile device is set at random in 28 bits of 4 octets (32 bits) according to ARIB-STD (Association of Radio Industries and Businesses Standard). It is not likely that the same link-identification codes are transmitted from two mobile devices in normal communications, as long as a maximum of two vehicles are within the communication service area.
It is however a recent trend to expand the communication service area so that one immobile device communicates with a plurality of mobile devices at the same time. The DSRC method defines a xe2x80x9cclass 2xe2x80x9d communication distance to be up to 30 meters. This 30-meter communication service area is considered to cover three traffic lanes. In this instance, it is estimated that a maximum of more than eight vehicles travel at the same time in the 30-meter communication service area.
It is herein assumed that one vehicle newly enters the 30-meter communication service area in addition to preceding eight vehicles having respective mobile communication devices, and transmits its link-identification codes. It is also assumed that the link-identifications codes LID#1 to LID#8 of the mobile devices of eight vehicles are different. The probability of coincidence of LID#9 of the mobile device of the ninth vehicle with LID#1 to LID#8 is calculated as follows.
First, the maximum possible number of LIDs is 228, because each link-identification is comprised of 28 bits. The probability (P1) of no coincidence of LID#9 with LID#1 to LID#9 is calculated as follows, in consideration that already eight LIDs are used.
P1=(228xe2x88x928)/228xe2x80x83xe2x80x83(1)
It is further assumed that a new vehicle enters the 30-meter communication service area every one second. In this instance, the probability (P2) of no coincidence among LIDs for a day (24 hours) is calculated as follows using the above expression (1).
P2=P1(24xc3x9760xc3x9760)=0.9974xe2x80x83xe2x80x83(2)
As a result, the probability of coincidence among LIDs for a day is about 0.26%.
It is a general practice to install the immobile devices at a plurality of locations along the road. For instance, the immobile device is provided at each of a hundred toll gates of an expressway. In this instance, the probability (P3) of no coincidence among the LIDs for a day throughout the expressway is calculated as follows.
P3=P2100=0.773xe2x80x83xe2x80x83(3)
As a result, the probability of coincidence for a day throughout the express way is about 22.7%.
If the above coincidence of LIDs occurs, the immobile device is disabled to distinguish mobile devices which have the same LID within the communication service area. This may be countered by transmitting from the immobile device to the mobile devices having the same LID a command to replace the same LID with new LIDS. However, the mobile device which has been communicating normally will be disabled to continue the existing communication.
It is therefore an object of the present invention to provide an inter-vehicle communication system and method which avoids a coincidence of link-identification codes of two mobile communication devices in the same communication service area.
According to the present invention, a mobile communication device uses its own link identification code generated at random for executing a communication with an immobile communication device in a communication service area of the immobile device. When the own link identification code coincides with another link identification code which is already being used for a communication with another mobile device within the same communication service area. The mobile device responsively changes the link identification code so that a changed link identification code is used for the communication with the immobile device.